Paypal Express not working with membership plugin

I have verified the email address and paypal single transaction works. In Paypal express mode, I keep getting the error, "The link you have used to enter the PayPal system is invalid. Please review the link and try again.", whether in live or sandbox mode. Tried a number of paypal accounts and got the same result. Error happens whether I try to use a paypal address or, once on the paypal login screen, I select pay with credit card. Any ideas? Very frustrating problem.